1. Field of the Invention
The present invention relates to a vibration isolation table for supporting an object to be isolated from vibration such as a precision instrument while isolating the object from vibration which is transmitted from the floor to the vibration isolation table, and more specifically, relates to such a vibration isolation table, the natural vibration frequency of which in the horizontal direction is small.
2. Description of the Related Art
As an example of a vibration isolation table for preventing vibration from the floor from being transmitted to a precision instrument (e.g., an optical stepper or an electron microscope) which is supported by the vibration isolation table, a vibration isolation table which includes a stationary housing, a pressure-tight flexible member (diaphragm), a primary movable base, a bottomed hollow cylindrical member and a vibration-free base is known in the art. The stationary housing is provided on top thereof with an opening. The pressure-tight flexible member is positioned to close this opening in an airtight fashion to form a first pressure chamber. The primary movable base is coupled to a central portion of the pressure-tight flexible member. The bottomed hollow cylindrical member, the top end of which is formed as an open end, is formed integral with the primary movable base to extend downwards from a central portion of the primary movable base into the first pressure chamber. The vibration-free base includes a swingable rod which extends inside the bottomed hollow cylindrical member, and amount which is fixed to the upper end of the swingable rod. The lower end of the swingable rod is supported by a bottom part of the bottomed hollow cylindrical member so that the swingable rod can swing with respect to the bottomed hollow cylindrical member.
If the floor on which the vibration isolation table is placed vibrates horizontally with a precision instrument being mounted on the mount of the vibration-free base, the swingable rod swings (oscillates) with the bottom end thereof, which is in contact with a bottom part of the bottomed hollow cylindrical member to be supported thereby, being the center of oscillation, to prevent the horizontal vibration of the floor from being transmitted to the precision instrument. This greatly reduces the influence of the horizontal vibration on the precision instrument.
In the case where the floor on which the precision instrument is placed vibrates vertically, some elements of the vibration isolation table such as the primary movable base and the swingable rod move up and down with respect to the stationary housing to prevent the vertical vibration of the floor from being transmitted to the precision instrument.
In such a vibration isolation table, the natural vibration frequency thereof in the horizontal direction is reduced to thereby improve the performance of a horizontal vibration isolating operation of the vibration isolation table by making an adjustment to the length of the bottomed hollow cylindrical member and the length of the swingable rod.
An improved vibration isolation table in which the natural vibration frequency thereof in the horizontal direction is further reduced to thereby further improve the performance of the horizontal vibration isolating operation has been desired in recent years. However, such an improved vibration isolation table cannot be achieved simply by making an adjustment to the length of the bottomed hollow cylindrical member and the length of the swingable rod.